1. Field of the Invention
This invention relates to a method for erasing a radiation image remaining in a stimulable phosphor sheet which has stored a radiation image and has been irradiated with stimulating rays to read the radiation image, by exposing the phosphor sheet to an erasing light, and a device employed in the method for erasing the radiation image.
2. Description of Prior Art
Certain phosphors absorb a portion of radiation energy when exposed to a radiation (e.g., X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, ultraviolet rays and electron beam), and give stimulated emission depending upon the amount of stored energy when irradiated with stimulating rays such as visible light. A phosphor showing such property is referred to as stimulable phosphor. There have been already known various stimulable phosphors. As representative examples of the stimulable phosphors, there can be mentioned a barium halide phosphor activated by a rare earth element such as europium, and an oxyhalide phosphor activated by a rare earth element such as cerium. Further, stimulable phosphors in which various additives are incorporated are also known.
A radiation storage sheet is, for instance, prepared by forming the above stimulable phosphor in a shape of a sheet with or without a binder. As a radiation image recording and reproducing method, there is proposed a method that a radiation image of high quality for diagnosis is obtained by processing the above stimulable sheet. In more detail, the method involves the steps of recording information with respect to human body in the radiation stimulable sheet, sequentially scanning the sheet with stimulating rays to release the radiation energy stored in the phosphor as light emission (stimulated emission); photo-electrically detecting the stimulated emission to obtain image signals; and obtaining the radiation image of high quality for diagnosis by processing of the image signals.
In the above radiation image recording and reproducing method, a wavelength region of stimulating rays and a wavelength region of stimulated emission should be separated. For instance, in order to detect effectively an extremely weak light of stimulated emission, a stimulated emission of wavelength within the range of 300 to 500 nm is preferably detected using stimulating rays of wavelength within the range of 600 to 700 nm. Hence, a stimulable phosphor is preferably selected to emit stimulated emission of wavelength within the range of 300 to 500 nm when excited with stimulating rays of wavelength within the range of 600 to 700 nm.
The finally obtained image can be reproduced on hard copy (e.g., in the form of printed image or photographic image), or reproduced on a screen of CRT (Cathode Ray Tube). The stimulable phosphor sheets can be formed in various shapes such as sheet, belt and drum. In the present specification, the term of "sheet" is used to include materials of all of these shapes.
The radiation image stored in the stimulable phosphor sheet can be erased, and therefore the stimulable phosphor sheet has the advantage that it can be used repeatedly. Accordingly, in the radiation image recording and reproducing method, the storage phosphor sheet is generally used repeatedly. In the radiation image recording and reproducing method, if the stimulable phosphor sheet is irradiated with stimulating rays having a sufficiently high energy to read the radiation image, a radiation energy of the stored radiation image information is released completely from the sheet. In practice, however, the stimulating rays used in the reading procedure can release only a portion of the stored radiation image. Thus, in the case of using the stimulable phosphor sheet repeatedly, a remaining portion of the recorded radiation image gives noise in a subsequently recorded radiation image.
Another problem resides in that a stimulable phosphor contains a small amount of radioactive isotopes such as .sup.226 Ra and .sup.40 K and the isotopes emit radiation. The radiation energy emitted by the isotopes is stored in the stimulable phosphor sheet when the phosphor sheet is allowed to stand, and such stored energy also gives noise. Further, the stimulable phosphor stores radiation energy of environmental radiation such as cosmic rays or radiation from isotope in environment. Such radiation energy stored in the stimulable phosphor sheet during standing (referred to as "fog") also gives noise in a subsequently recorded radiation image. Accordingly, the fog should be also erased.
In the radiation image recording and reproducing method which uses the stimulable phosphor sheet repeatedly, it is required to prevent noise caused by the unreleased portion of the previously recorded radiation image, as well as noise caused by the fog. Then, there is already known a method for erasing a remaining radiation image by exposing a stimulable phosphor to light containing a light portion of wavelength region corresponding to that of the stimulating rays to release sufficiently the remaining radiation energy before initiating the next procedure for recording radiation image in the stimulable phosphor sheet.
As the erasing methods, there are known various methods such as a method of using a light source emitting light having a relatively long wavelength (e.g., a tungsten lamp emitting light of a wavelength region of visible light to infrared rays, a halogen lamp or an infrared lamp) as described in Japanese Patent Provisional Publication No. 56(1981)-11392, a method of using a light source emitting light having a relatively short wavelength (e.g., a fluorescent lamp, laser beam source, a sodium lamp, a neon lamp, metal halide lamp or a xenon lamp) as described in Japanese Patent Provisional Publication No. 58(1983)-83839, and a method of conducting the erasing procedure twice comprising a second erasing procedure of exposing a stimulable phosphor sheet having been subjected to a first erasing procedure to light in an amount of light of a ratio of 1/5 to 3/10,000 to an amount of light of a first erasing procedure just before initiating the next recording procedure.
Japanese Patent Provisional Publication No. 59(1984)-202099 proposes a method for erasing the remaining radiation energy by exposing a stimulable phosphor sheet to light of a spectrum having both absorption wavelength and stimulated wavelength of the phosphor. The publication discloses a method of for erasing a remaining radiation energy by exposing a stimulable phosphor sheet to an erasing light containing a light portion in the region of absorption wavelength and stimulated wavelength of the phosphor, and a method for erasing a remaining radiation energy by exposing a stimulable phosphor sheet to the above erasing light and then exposing to a second erasing light, in which a short wavelength not more than 500 nm of the erasing light is cut off by a color glass filter, in the same amount of light as that of the erasing light.